Because of the lower aerodynamic drag at high altitudes, a higher cruising altitude of an aircraft fundamentally allows a reduction of the fuel consumption of the aircraft engines. There is therefore a trend towards designing commercial aircraft for flights at increasingly high cruising altitudes of up to 43,000 feet (ca. 13100 m). Commercial aircraft currently in operation comprise a pressurized cabin, the internal pressure of which during cruising is held by means of an air-conditioning system, which is supplied with engine bleed air, at a pressure level that is higher than the ambient pressure, i.e. the reduced atmospheric pressure at high altitudes.
In order to limit the fuselage loads resulting from the pressure difference between the ambient pressure and the higher cabin internal pressure as well as the work that the air-conditioning system has to do in order to maintain the desired pressure level in the interior of the aircraft cabin, the cabin internal pressure of a commercial aircraft during cruising of the aircraft is not held at a pressure level corresponding to the atmospheric pressure at sea level. Rather, the pressure in the interior of the cabin of a commercial aircraft during cruising of the aircraft, i.e. when the aircraft is situated at cruising altitude, usually corresponds approximately to the atmospheric pressure at a height of 8000 feet (ca. 2400 m).
In principle, the oxygen partial pressure of the air in the interior of the aircraft cabin that arises in the case of a cabin internal pressure corresponding approximately to the atmospheric pressure at a height of 8000 feet is high enough to guarantee an adequate supply of oxygen for the crew and passengers on board the aircraft. However, compensating for the pressure difference between the cabin internal pressure during cruising of the aircraft, which corresponds approximately to the atmospheric pressure at a height of 8000 feet, and the pressure level on the ground, which usually corresponds to or lies slightly below the atmospheric pressure at sea level, requires the performance of an adaptation process by the human body. Particularly for persons who react sensitively to a reduced oxygen partial pressure, the performance of this adaptation process may be taxing and lead to symptoms such as dizziness or even fainting. These symptoms may be additionally intensified by some other illness of an affected person or by stress.
Besides the reduced oxygen partial pressure, the low air humidity in the interior of an aircraft cabin may lead to health problems in sensitively reacting persons. In order to minimize the condensing-out of water on the cold outer skin of the fuselage of the aircraft, the air humidity in the aircraft cabin is kept artificially low and in a front fuselage portion of the aircraft is only approximately 5%. Such a low air humidity is generally perceived as uncomfortable and may lead to drying-out and irritation of the mucous membranes.
As a result of the reduced oxygen partial pressure and the low moisture content of the breathing air in the interior of the aircraft cabin, the crew and the passengers of a commercial aircraft during the flight are subject as a whole to increased physical stress. The extent to which a person perceives this stress however depends upon the individual sensitivity of the person to the special ambient conditions on board an aircraft and upon the flying time. The negative effects of the increased physical stress are therefore perceived to a greater extent by persons who react sensitively to a reduced oxygen partial pressure and low air humidity, especially on long-haul flights.
For the improved comfort of crew members and passengers on board an aircraft and in order to prevent health problems that are caused by a reduced oxygen partial pressure in an aircraft cabin, EP 0 808 769 A2 proposes to supply air which is oxygenated by means of an oxygen concentrator to an aircraft cabin. With the aid of the system described in this document the oxygen content of the air to be supplied to the aircraft cabin may be increased to 25 to 35% by volume. The oxygen enrichment system known from EP 0 808 769 A2 makes it possible to increase the well-being of the crew members and the passengers on board the aircraft. What is more, the pressure difference between the ambient pressure during the flight and the raised cabin internal pressure and hence the stresses of the fuselage resulting therefrom may be limited because the raising of the oxygen partial pressure in the aircraft cabin is realized exclusively by means of oxygen enrichment of the air to be supplied to the cabin and not by means of an increase of the cabin pressure.
An important drawback of the system described in EP 0 808 769 A2 is however that in order to increase the oxygen partial pressure in the aircraft cabin a large quantity of oxygen has to be produced. The oxygen concentrator, which is operated permanently under a high load, therefore involves a high maintenance outlay. The individual components of the system as well as the entire system are moreover very heavy. If instead of the oxygen concentrator described in EP 0 808 769 A2 an oxygen reservoir is used to provide the oxygen needed to enrich the air to be supplied to the aircraft cabin, this oxygen reservoir is likewise very heavy and moreover takes up a large amount of installation space.
The underlying object of the invention is therefore to provide a lightweight and compact device for improving the breathing air quality in an aircraft cabin.